How to re-render animation?
First, you did the right thing by rendering into an image sequence. Never render animations directly as video.

Delete (or move to a different folder) the frames that need to be re-rendered. Then on the output settings uncheck the "overwrite option"



Then re-render as usual wiht CtrlF12. Blender will just render the missing frames and leave the others untouched.

If you have any strips on the VSE (video sequence editor) uncheck the Sequencer box of the postprocessing section of the properties window.